Interfacial control of mineral fillers in vat photopolymerization: effects on dispersion and mechanical response

In the field of additive manufacturing (AM), Vat Photopolymerization (VP) excels in high resolution and in the ability to produce complex structures, ideal for applications in biomedicine, electronics, and rapid prototyping. However, pure photopolymer resins have mechanical limitations, such as brittleness and low elastic modulus. Adding mineral fillers offers an effective way to address these limitations, but it demands careful control over dispersion and interfacial interactions to prevent aggregate buildup. This study investigates the role of interfacial chemistry in determining dispersion, matrix compatibility, and final mechanical response. The dispersion and ultimate mechanical properties of two different mineral fillers (SiO₂ size ≈ 50 μm, and CaCO₃ size ≈ 1 μm) both unmodified and functionalized with octadecyltrimethoxysilane, were evaluated across various formulations, both with and without plasticizer and/or dispersing agent. After careful comparison the formulations that demonstrated a simultaneous improvement across all evaluated mechanical properties in plasticizer-containing formulations were the ones loaded at 20% with functionalized silica and calcium carbonate, with respective improvements in elastic modulus of + 16% and + 11%, in elongation at break of + 12% and + 21%, and in tensile strength of + 25% and + 12%. Whereas in plasticizer-free formulations, the formulations that demonstrated a simultaneous improvement across all evaluated mechanical properties were those loaded at 10 wt% with functionalized silica and unmodified calcium carbonate, with respective improvements in elastic modulus of + 232% and + 202%, elongation at break of + 49% and + 112%, and tensile strength of + 127% and + 180%. These results provide useful guidelines for advanced VP resins, positioning surface functionalization as a key approach to optimize photopolymer composites.